Archimedes’ Principle is applicable to gases as well as to liquids. We know that a liquid exerts upthrust on a body immersed in it. Similarly, a gas also exerts upthrust on a body immersed in it and the body appears to lose a part of its weight. This loss in weight is equal to the weight of the gas displaced by the body.
For this reason, the weight of a body in the air becomes slightly less than its weight in a vacuum. But due to the negligible upthrust of air, this loss of weight is also negligibly small and hence we often ignore the buoyant force offered by air.
The Upward motion of a balloon: The upward motion of a balloon depends on the upthrust of air. The net. weight of an inflated balloon containing hydrogen gas is much less than the weight of air displaced by it.
As a result, the buoyant force exerted by the air on the balloon becomes greater than the weight of the balloon. Due to this reason, the balloon experiences a net upward force and it moves up. We know that with the increase in attitude from the surface of the earth, the density of air decreases, and hence the upthrust exerted on the balloon decreases. After attaining a certain height, the weight of the balloon becomes equal to the buoyant force acting on it and then the balloon can’t rise any further, it remains floating at a particular altitude.